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<tr class="t-nv-h2"><td colspan="5"> <code>this_thread</code> 命名空间</td></tr>
<tr class="t-nv-col-table"><td><div><table class="t-nv-begin" cellpadding="0" style="">
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<tr class="t-nv-col-table"><td><div><table class="t-nv-begin" cellpadding="0" style="">
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<tr class="t-nv-h2"><td colspan="5"> 通用锁管理</td></tr>
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<tr class="t-nv-h1"><td colspan="5"> 条件变量</td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="condition_variable.html" title="cpp/thread/condition variable"><span class="t-lines"><span>condition_variable</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="condition_variable_any.html" title="cpp/thread/condition variable any"><span class="t-lines"><span>condition_variable_any</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="notify_all_at_thread_exit.html" title="cpp/thread/notify all at thread exit"><span class="t-lines"><span>notify_all_at_thread_exit</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="cv_status.html" title="cpp/thread/cv status"><span class="t-lines"><span>cv_status</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div></div></td></tr>
<tr class="t-nv-h1"><td colspan="5"> 信号量</td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="counting_semaphore.html" title="cpp/thread/counting semaphore"><span class="t-lines"><span>counting_semaphore</span><span>binary_semaphore</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx20">(C++20)</span></span><span><span class="t-mark-rev t-since-cxx20">(C++20)</span></span></span></div></div></td></tr>
<tr class="t-nv-h1"><td colspan="5"> 闩与屏障</td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="latch.html" title="cpp/thread/latch"><span class="t-lines"><span>latch</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx20">(C++20)</span></span></span></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="barrier.html" title="cpp/thread/barrier"><span class="t-lines"><span>barrier</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx20">(C++20)</span></span></span></div></div></td></tr>
<tr class="t-nv-h1"><td colspan="5"> future</td></tr>
<tr class="t-nv-col-table"><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="promise.html" title="cpp/thread/promise"><span class="t-lines"><span>promise</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="future.html" title="cpp/thread/future"><span class="t-lines"><span>future</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="shared_future.html" title="cpp/thread/shared future"><span class="t-lines"><span>shared_future</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="packaged_task.html" title="cpp/thread/packaged task"><span class="t-lines"><span>packaged_task</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="async.html" title="cpp/thread/async"><span class="t-lines"><span>async</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div></div></td></tr>
</table></div></td><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="launch.html" title="cpp/thread/launch"><span class="t-lines"><span>launch</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="future_status.html" title="cpp/thread/future status"><span class="t-lines"><span>future_status</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="future_error.html" title="cpp/thread/future error"><span class="t-lines"><span>future_error</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="future/future_category.html" title="cpp/thread/future category"><span class="t-lines"><span>future_category</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="future_errc.html" title="cpp/thread/future errc"><span class="t-lines"><span>future_errc</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div></div></td></tr>
</table></div></td></tr>
</table></div><div></div></div></div></div><div class="t-navbar-sep"> </div></div>
<table class="t-dcl-begin"><tbody>
<tr class="t-dsc-header">
<td> <div>定义于头文件 <code><a href="../header/mutex.html" title="cpp/header/mutex">&lt;mutex&gt;</a></code>
 </div></td>
<td></td>
<td></td>
</tr>
<tr class="t-dcl t-since-cxx11">
<td> <div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1">&lt;</span> <span class="kw1">class</span> Callable, <span class="kw1">class</span>... <span class="me1">Args</span> <span class="sy1">&gt;</span><br>
<span class="kw4">void</span> call_once<span class="br0">(</span> <a href="once_flag.html"><span class="kw2174">std::<span class="me2">once_flag</span></span></a><span class="sy3">&amp;</span> flag, Callable<span class="sy3">&amp;&amp;</span> f, Args<span class="sy3">&amp;&amp;</span>... <span class="me1">args</span> <span class="br0">)</span><span class="sy4">;</span></span></div></td>
<td class="t-dcl-nopad">  </td>
<td> <span class="t-mark-rev t-since-cxx11">(C++11 起)</span> </td>
</tr>
<tr class="t-dcl-sep"><td></td><td></td><td></td></tr>
</tbody></table>
<p>准确执行一次<a href="../named_req/Callable.html" title="cpp/named req/Callable"><i>可调用</i> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(Callable)</span> </a>对象 <code>f</code> ，即使同时从多个线程调用。
</p><p>细节为：
</p>
<ul><li> 若在调用 <code>call_once</code> 的时刻， <code>flag</code> 指示已经调用了 <code>f</code> ，则 <code>call_once</code> 立即返回（称这种对 <code>call_once</code> 的调用为<i>消极</i>）。
</li></ul>
<ul><li> 否则， <code>call_once</code> 以参数 <span class="t-c"><span class="mw-geshi cpp source-cpp">std​<span class="sy4">::</span>​forward<span class="sy1">&lt;</span>Args<span class="sy1">&gt;</span><span class="br0">(</span>args<span class="br0">)</span>...</span></span> 调用 <span class="t-c"><span class="mw-geshi cpp source-cpp">​std​<span class="sy4">::</span>​forward<span class="sy1">&lt;</span>Callable<span class="sy1">&gt;</span><span class="br0">(</span>f<span class="br0">)</span></span></span> （如同用 <span class="t-lc"><a href="../utility/functional/invoke.html" title="cpp/utility/functional/invoke">std::invoke</a></span> ）。不同于 <span class="t-lc"><a href="thread.html" title="cpp/thread/thread">std::thread</a></span> 构造函数或 <span class="t-lc"><a href="async.html" title="cpp/thread/async">std::async</a></span> ，不移动或复制参数，因为不需要转移它们到另一执行线程（称这种对 <code>call_once</code> 的调用为<i>积极</i>）。
</li></ul>
<dl><dd><ul><li> 若该调用抛异常，则传播异常给 <code>call_once</code> 的调用方，并且不翻转 <code>flag</code> ，以令其他调用将得到尝试（这种对 <code>call_once</code> 的调用被称为<i>异常</i>）。
</li><li> 若该调用正常返回（这种对 <code>call_once</code> 的调用被称为<i>返回</i>），则翻转 <code>flag</code> ，并保证以同一 <code>flag</code> 对 <code>call_once</code> 的其他调用为<i>消极</i>。
</li></ul>
</dd></dl>
<p>同一 <code>flag</code> 上的所有<i>积极</i>调用组成单独全序，它们由零或多个<i>异常</i>调用后随一个<i>返回</i>调用组成。该顺序中，每个<i>积极</i>调用的结尾同步于下个<i>积极</i>调用。
</p><p>从<i>返回</i>调用的返回同步于同一 <code>flag</code> 上的所有<i>消极</i>调用：这表示保证所有对 <code>call_once</code> 的同时调用都观察到<i>积极</i>调用所做的任何副效应，而无需额外同步。
</p>
<h3><span class="mw-headline" id=".E5.8F.82.E6.95.B0">参数</span></h3>
<table class="t-par-begin">


<tr class="t-par">
<td>  flag
</td>
<td> -
</td>
<td> 对象，对于它只有一个函数得到执行
</td></tr>
<tr class="t-par">
<td>  f
</td>
<td> -
</td>
<td> 要调用的<a href="../named_req/Callable.html" title="cpp/named req/Callable"><i>可调用</i> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(Callable)</span> </a>对象
</td></tr>
<tr class="t-par">
<td>  args...
</td>
<td> -
</td>
<td> 传递给函数的参数
</td></tr></table>
<h3><span class="mw-headline" id=".E8.BF.94.E5.9B.9E.E5.80.BC">返回值</span></h3>
<p>（无）
</p>
<h3><span class="mw-headline" id=".E5.BC.82.E5.B8.B8">异常</span></h3>
<ul><li> 若任何条件阻止对 <code>call_once</code> 的调用按规定执行，则抛出 <span class="t-lc"><a href="../error/system_error.html" title="cpp/error/system error">std::system_error</a></span>
</li><li> 任何 <code>f</code> 所抛的异常
</li></ul>
<h3><span class="mw-headline" id=".E6.B3.A8.E8.A7.A3">注解</span></h3>
<p>若对 <code>call_once</code> 的同时调用传递不同的 <code>f</code> ，则调用哪个 <code>f</code> 是未指定的。被选择函数运行于与传递它的 <code>call_once</code> 的调用相同的线程。
</p><p>即使在从多个线程调用时，也保证<a href="../language/storage_duration.html#.E9.9D.99.E6.80.81.E5.B1.80.E9.83.A8.E5.8F.98.E9.87.8F" title="cpp/language/storage duration">函数局域静态对象</a>的初始化仅出现一次，这可能比使用 <code>std::call_once</code> 的等价代码更为高效。
</p><p>此函数的 POSIX 类似物是 <a rel="nofollow" class="external text" href="http://pubs.opengroup.org/onlinepubs/9699919799/functions/pthread_once.html">pthread_once</a> 。
</p>
<h3><span class="mw-headline" id=".E7.BC.BA.E9.99.B7.E6.8A.A5.E5.91.8A">缺陷报告</span></h3>
<p>下列更改行为的缺陷报告追溯地应用于以前出版的 C++ 标准。
</p>
<table class="dsctable" style="font-size:0.8em">
<tr>
<th> DR
</th>
<th> 应用于
</th>
<th> 出版时的行为
</th>
<th> 正确行为
</th></tr>
<tr>
<td> <a rel="nofollow" class="external text" href="https://cplusplus.github.io/LWG/issue2442">LWG 2442</a>
</td>
<td> C++11
</td>
<td> 在调用前复制并/或移动参数
</td>
<td> 不进行复制/移动
</td></tr></table>
<h3><span class="mw-headline" id=".E7.A4.BA.E4.BE.8B">示例</span></h3>
<div class="t-example"><div class="t-example-live-link"><div class="coliru-btn coliru-btn-run-init">运行此代码</div></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="co2">#include &lt;iostream&gt;</span>
<span class="co2">#include &lt;thread&gt;</span>
<span class="co2">#include &lt;mutex&gt;</span>
 
<a href="once_flag.html"><span class="kw2174">std::<span class="me2">once_flag</span></span></a> flag1, flag2<span class="sy4">;</span>
 
<span class="kw4">void</span> simple_do_once<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
    std<span class="sy4">::</span><span class="me2">call_once</span><span class="br0">(</span>flag1, <span class="br0">[</span><span class="br0">]</span><span class="br0">(</span><span class="br0">)</span><span class="br0">{</span> <a href="../io/cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">"Simple example: called once<span class="es1">\n</span>"</span><span class="sy4">;</span> <span class="br0">}</span><span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span>
 
<span class="kw4">void</span> may_throw_function<span class="br0">(</span><span class="kw4">bool</span> do_throw<span class="br0">)</span>
<span class="br0">{</span>
  <span class="kw1">if</span> <span class="br0">(</span>do_throw<span class="br0">)</span> <span class="br0">{</span>
    <a href="../io/cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">"throw: call_once will retry<span class="es1">\n</span>"</span><span class="sy4">;</span> <span class="co1">// 这会出现多于一次</span>
    <span class="kw1">throw</span> <a href="../error/exception.html"><span class="kw764">std::<span class="me2">exception</span></span></a><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
  <span class="br0">}</span>
  <a href="../io/cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">"Didn't throw, call_once will not attempt again<span class="es1">\n</span>"</span><span class="sy4">;</span> <span class="co1">// 保证一次</span>
<span class="br0">}</span>
 
<span class="kw4">void</span> do_once<span class="br0">(</span><span class="kw4">bool</span> do_throw<span class="br0">)</span>
<span class="br0">{</span>
  <span class="kw1">try</span> <span class="br0">{</span>
    std<span class="sy4">::</span><span class="me2">call_once</span><span class="br0">(</span>flag2, may_throw_function, do_throw<span class="br0">)</span><span class="sy4">;</span>
  <span class="br0">}</span>
  <span class="kw1">catch</span> <span class="br0">(</span>...<span class="br0">)</span> <span class="br0">{</span>
  <span class="br0">}</span>
<span class="br0">}</span>
 
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
    <a href="thread.html"><span class="kw2146">std::<span class="me2">thread</span></span></a> st1<span class="br0">(</span>simple_do_once<span class="br0">)</span><span class="sy4">;</span>
    <a href="thread.html"><span class="kw2146">std::<span class="me2">thread</span></span></a> st2<span class="br0">(</span>simple_do_once<span class="br0">)</span><span class="sy4">;</span>
    <a href="thread.html"><span class="kw2146">std::<span class="me2">thread</span></span></a> st3<span class="br0">(</span>simple_do_once<span class="br0">)</span><span class="sy4">;</span>
    <a href="thread.html"><span class="kw2146">std::<span class="me2">thread</span></span></a> st4<span class="br0">(</span>simple_do_once<span class="br0">)</span><span class="sy4">;</span>
    st1.<span class="me1">join</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
    st2.<span class="me1">join</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
    st3.<span class="me1">join</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
    st4.<span class="me1">join</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
 
    <a href="thread.html"><span class="kw2146">std::<span class="me2">thread</span></span></a> t1<span class="br0">(</span>do_once, <span class="kw2">true</span><span class="br0">)</span><span class="sy4">;</span>
    <a href="thread.html"><span class="kw2146">std::<span class="me2">thread</span></span></a> t2<span class="br0">(</span>do_once, <span class="kw2">true</span><span class="br0">)</span><span class="sy4">;</span>
    <a href="thread.html"><span class="kw2146">std::<span class="me2">thread</span></span></a> t3<span class="br0">(</span>do_once, <span class="kw2">false</span><span class="br0">)</span><span class="sy4">;</span>
    <a href="thread.html"><span class="kw2146">std::<span class="me2">thread</span></span></a> t4<span class="br0">(</span>do_once, <span class="kw2">true</span><span class="br0">)</span><span class="sy4">;</span>
    t1.<span class="me1">join</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
    t2.<span class="me1">join</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
    t3.<span class="me1">join</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
    t4.<span class="me1">join</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span></pre></div></div>
<p>可能的输出：
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="text source-text"><pre class="de1">Simple example: called once
throw: call_once will retry
throw: call_once will retry
Didn't throw, call_once will not attempt again</pre></div></div> 
</div>
<h3><span class="mw-headline" id=".E5.8F.82.E9.98.85">参阅</span></h3>
<table class="t-dsc-begin">

<tr class="t-dsc">
<td>  <div class="t-dsc-member-div"><div><a href="once_flag.html" title="cpp/thread/once flag"> <span class="t-lines"><span>once_flag</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div></div>
</td>
<td>  确保 <strong class="selflink"><tt>call_once</tt></strong> 只调用函数一次的帮助对象 <br> <span class="t-mark">(类)</span> </td></tr>

</table>

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